Long-billed Curlew (Numenius americanus): a technical Conservation Assessment Prepared for the usda forest Service, Rocky Mountain Region, Species Conservation Project December 12, 2006 James A. Sedgwick
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Figur e 12c. Predators/competitors centrum of the long-billed curlew envirogram. Web 4 Web 3 Web 2 Web 1 CENTRUM P re da to rs /C om pe ti to rs
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34 35 Grazing The major historical threat to long-billed curlews was the removal of primary, native grazers (i.e., bison [Bison bison], pronghorn [Antilocapra
the mixed-grass and shortgrass prairies from the once-heterogeneous, patchy grassland landscape that resulted from the intense, uneven grazing by these species. Today cattle and sheep have replaced bison, but they do not mimic the historical grazing patterns of these native herbivores. Because long-billed curlews are found across such a wide range of climate regimes, from more xeric in the southern parts of their range to more mesic in the north, the grassland prairie systems that they occupy express a similar diversity in plant species composition and variety, in vegetation height and density, and in growth form. As a result, one might expect a congruent variation, from xeric to mesic, in plant species’ response to grazing and in grazing impacts on curlew habitats. Optimal grazing intensity and appropriate grazing regimes vary according to prairie type and climate regimes. Grazing generally enhances curlew breeding habitats because it produces the short grass and open ground that curlews favor for predator detection and chick mobility (e.g., King 1978, Pampush 1980, Jenni et al. 1981, Bicak et al. 1982, Cochrane and Anderson 1987, Hooper and Pitt 1996). In the Northern Great Plains, highest curlew densities occurred in lightly grazed grasslands on dry soils, and in heavily grazed areas on moister soils (Kantrud and Kologiski 1982). In Idaho, curlew numbers were positively correlated with grazing intensity (Bicak et al. 1982), and breeding was never observed on rangeland that had not been grazed in the previous 12 months (Jenni et al. 1981). In Nebraska, curlews were only observed on grazed areas (Cole and Sharpe 1976) and only in summer-grazed fields (Bicak 1977). In British Columbia, highest breeding densities occurred on sites where spring grazing levels averaged 1.4 animal units per ha (Hooper and Pitt 1996). In habitats with dense stands of perennial bunchgrass, sheep are better at trampling residual vegetation and creating appropriate breeding habitat than cattle are (Jenni et al. 1981, Bicak et al. 1982). In southwestern Idaho, areas grazed by sheep alone or sheep and cattle had higher densities of curlews than did areas grazed by cattle alone (Bicak et al. 1982). A year-round grazing schedule was least attractive to breeding curlews in Idaho, and rest-rotation systems that rested pastures from March through May were also detrimental (Redmond and Jenni 1982). The best rotation system included grazing through early spring, so vegetation height and density were low during courtship and egg laying (Jenni et al. 1981). Overgrazing in drier, shortgrass habitats may be a threat to long-billed curlews and should be avoided (Strong 1971, Bock et al. 1993, Anstey et al. 1995). Areas where vegetation is already sparse and short from overgrazing should be protected to improve their condition (Oberholser 1974). Grazing in more mesic, mixed-grass habitats may benefit long-billed curlews (Kantrud and Kologiski 1982, Messmer 1990). Mixed- grass areas or areas where the grass is too tall or thick can be made suitable for breeding long-billed curlews by implementing moderate grazing (Dechant et al. 2003). Grazing moister areas will increase vegetation diversity and patchiness and reduce tall, thick vegetation (Kantrud and Kologiski 1982). In such habitats, some grazing appears to benefit this species. Fire and fire suppression The fragmentation of the mixed-grass and shortgrass prairies by agricultural conversion has prevented extensive, uncontrolled wildfires, and those that do occur are often contained to the smallest area possible (Bent 1929, Samson and Knopf 1994, Risser 1996). Long-billed curlews may benefit from wildfires on grassland habitats during late summer (Cannings 1999). Burning can improve habitat for curlews by removing shrubs and increasing habitat openness (Pampush and Anthony 1993). Fire suppression may negatively affect breeding habitat by allowing forest encroachment and growth of tall grasses and shrubs (Cannings 1999). During the breeding season following a fall range fire, there was a 30 percent increase in estimated curlew breeding density in western Idaho (Redmond and Jenni 1986). Plant succession following fire can be rapid, so grazing or mowing must also be used to maintain burned areas as attractive breeding habitat for curlews (Jenni et al. 1981). Exotic species Early attempts to rehabilitate grasslands included seeding with exotic crested wheatgrasses imported from Siberia and planting trees to control wind erosion (implemented by the Civilian Conservation Corp from 1938 to 1941) (Samson and Knopf 1994). Prairie restoration efforts that seeded degraded grasslands with taller, exotic grasses have reduced habitat quality for grassland nesting birds (Samson and Knopf 1994). Throughout their range, long-billed curlews prefer native Yüklə 1,74 Mb. Dostları ilə paylaş:
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